Tracer ammunition allows one to track the trajectory of one or more bullets that one has just fired so that one can adjust one's aim. Historically, tracer ammunition has used pyrotechnic material (such as phosphorous or magnesium) that is ignited as the bullet is fired, and glows or emits visible light that can be seen as the bullet travels.
Unfortunately, using such tracer ammunition causes problems. One of the biggest problems is that the burning pyrotechnic material is visible in all directions. This enables enemy combatants to back-trace the path of the tracer bullet to its origin, and thus exposes the position of the shooter to enemy combatants. Also, allied combatants, such as all the combatants in a company, may have trouble distinguishing their own tracers from their allies' tracers. In addition, with pyrotechnic material the mass and density of the tracer bullet is different than the other non-tracer bullets that the tracer's trajectory is supposed to mimic, and thus does not accurately show the trajectory of the non-tracer bullet. This is true when the tracer is first fired out of a gun because the pyrotechnic material does not have the same mass as the material of the bullet (often lead) that the pyrotechnic material replaces. This is also true while the tracer travels down range because as the pyrotechnic material burns, the mass of pyrotechnical material decreases. This change in mass affects the ballistics of the tracer and can cause the shooter to misjudge the trajectory of the non-tracer bullets. Finally, pyrotechnic material creates a fire hazard upon impact, which may create problems during a training exercise or other similar situations.
Some tracer ammunition reduces the visible light component of the pyrotechnic material so that the tracer is only visible via a night-vision goggle (NVG). This solution works when an enemy combatant does not have a night-vision goggle, but does not work well in daylight. Furthermore, many consumer-grade cameras can see in the near-infrared spectrum (where NVGs operate) and thus allow enemy combatants to track such tracers.
Other tracers include an LED or diode laser, power source, and switch mounted onto a bullet to emit light backwards toward the shooter. But such tracers are difficult and complex to make because the light source, power source and switch are bulky relative to the bullet that they're mounted onto. This bulk also dramatically changes the ballistics of the tracer which causes the tracer's trajectory to not mimic the non-tracer's trajectory. In addition, the light emitted from the LED or diode laser is often visible from locations remote from the shooter. This is especially true when the tracer wobbles or tumbles during flight.